System, device, and method for over-current relays protecting motors

Abstract

A system, method, and device for protecting an induction motor are disclosed. The exemplary system may have a module for determining the current drawn by the motor and a module for determining the state of the motor. The system may calculate a used thermal capacity based on a first formula when the motor is in an active state. The system also calculates the used thermal capacity based on a second formula when the motor is in an inactive state. When the used thermal capacity attains a threshold, the relay is tripped thus removing current to the motor and preventing motor from overheating. A method to derive thermal time constants from desired trip time limits (such as those defined by IEC standards or the thermal limit curves provided by motor manufacturers) is also presented. For example, the thermal time constants can then be used in tacking motor used thermal capacity throughout various motor states.

Description

CROSS REFERENCE TO RELATED APPLICATIONS [0001] This application is related to U.S. Provisional Patent Application No. 60 / 727,464 filed Oct. 17, 2005 entitled “Thermal model based DSP algorithm for solid-state over current relays protecting AC motors”, which is incorporated fully herein by reference.TECHNICAL FIELD [0002] The present invention relates to controls and protection for motors and more particularly, to a device, method, and system for determining the thermal properties of a motor. BACKGROUND INFORMATION [0003] In current practice, there are two types of control algorithms for solid-state over-current relays, namely, electromechanical relay algorithm [1][2] and 12t algorithm. The electromechanical relay algorithm is derived from the model of electromechanical relay using the following: For⁢ ⁢0<M<1⁢⁢t⁡(I)⁢ =trM2-1A1For⁢ ⁢M>1⁢⁢t⁡(I)=AMP-1+BA2[0004] Where [0005] t(I) is the reset time in Eq. (A1) or trip time in Eq. (A2) in seconds, [0006] M is the Iinput / Ipickup (I...

AI Technical Summary

Benefits of technology

[0027] The present invention is a novel device, system, and method for an over-current relay protecting an induction motor comprising the following actions. The exemplary method may determine the current drawn by the motor. The method may also determine the state of the motor. The method may then calculate a used thermal capacity based on a first formula when the motor is in an active state. The method may also calculate the used thermal capacity based on a second formula when the motor is in an inactive state. Once the used thermal capacity attains a threshold the method may trip the relay, remove current from the motor, and prevent the motor from overheating.

Problems solved by technology

Therefore equation A1 does not simulate motor thermal dynamics for 0<M<1, which results in the algorithm not accurately tracking the used thermal capacity of AC motor under varying load.

From the analysis above, it is seen that both electromechanical relay algorithm and 12t algorithm may fail the thermal memory test and may not provide sufficient protection to AC induction motors.

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